1. Field of the Invention
The present invention relates to a process for producing a toner for developing an electrostatic latent image (electrostatic charge image) which may be formed by an image forming method such as electrophotography and electrostatic recording.
2. Related Background Art
Conventionally, an electrical or electrostatic latent image formed by an image forming apparatus such as electrophotographic apparatus and electrostatic recording apparatus is subsequently developed by use of a toner to form a toner image. The toner image is transferred onto a transfer material such as paper, as desired, and then is fixed onto the transfer material by at least one of various methods such as those utilizing heat, pressure and solvent vapor to provide a copied product.
Heretofore, a toner used for such a purpose has generally been manufactured by heat-fusing and mixing predetermined components such as colorant, charge controlling agent and anti-offset agent in a thermoplastic resin to uniformly disperse them in the resin so as to provide a composition, pulverizing the composition and then classifying the resultant pulverized product.
Such a toner preparation process ( i.e., pulverization process) is capable of producing a toner having good characteristics to a certain degree, but the materials for toner usable in the pulverization process has a certain limitation. That is, the selection of such materials is rather limited. For example, the composition produced by the above-mentioned heat-fusing and mixing step is required to be a species thereof which is capable of being pulverized and classified by means of an economically usable manufacturing system. In order to satisfy this requirement, the composition produced by the heat-fusing and mixing step must be formed as a sufficiently fragile product. For this reason, when the composition is actually intended to be pulverized, the resultant particles are liable to have a broad distribution in their particle sizes. Particularly, when a toner capable of providing a copied product with good resolution and good gradation characteristic is intended to be obtained, fine particles (e.g., those having a particle size of not more than 5 .mu.m) and coarse particles (e.g., those having a particle size of not less than 20 .mu.m) must be removed by classification. In such a case, the resultant yield is greatly decreased.
Further, in this pulverization process, it is difficult to uniformly disperse solid fine particles such as colorant, charge controlling agent and anti-offset agent in the above-mentioned thermoplastic resin. Therefore, sufficient attention must be paid to the degree of dispersion of the solid fine particles, because an increase in fog or a decrease in image density can be caused in the development of an electrostatic latent image depending on the degree of dispersion of the solid fine particles in the thermoplastic resin.
Nonuniformity in the dispersion of the solid fine particles in the thermoplastic resin resulting from the pulverization process considerably affects the properties of the resultant toner such as fluidity and triboelectric chargeability (or charging property) and has a marked effect on the characteristics of the toner such as developing property and durability.
On the other hand, in order to overcome the above-mentioned problems encountered in the conventional pulverization process, it has been proposed to produce a toner through suspension polymerization. In this suspension polymerization process, a polymerizable monomer composition comprising a polymerizable monomer and predetermined components such as a colorant, a charge controlling agent, an anti-offset agent and a polymerization initiator uniformly dissolved or dispersed in the monomer, is charged into water (or an aqueous dispersion medium mainly comprising water) containing a dispersion stabilizer (or suspension stabilizer), is dispersed and granulated in the aqueous dispersion medium by means of a mixing device capable of providing a high shearing force, and then the resultant dispersion is subjected to polymerization to form toner particles.
In the suspension polymerization process, a dispersion stabilizer is usually contained in a dispersion medium for the purpose of stable dispersion of droplets (i.e., particles of the polymerizable monomer composition) mainly comprising a polymerizable monomer before the polymerization and for the purpose of stable dispersion of the droplets or particles of the polymerized product during the polymerization.
The dispersion stabilizer is roughly classified into two groups of a water-soluble polymer which is capable of forming a protective colloid film to provide a repulsive force on the basis of steric hindrance, and a hardly water-soluble inorganic substance capable of providing an electrostatic repulsive force so as to stabilize the dispersion.
As the former, dispersion stabilizers comprising a water-soluble polymer such as polyvinyl alcohol, methyl cellulose and gelatin have been known. It has also been proposed to produce a toner through suspension polymerization by using one of these dispersion stabilizers. However, such a suspension polymerization method is liable to produce particles including a large amount of fine particles having a small particle size, and therefore the resultant particle size distribution becomes broad. In addition, it is difficult to remove the dispersion stabilizer attached to the surfaces of the resultant toner particles. Accordingly, the thus produced toner is liable to have considerably inferior electrical characteristics (e.g., electrical resistance chargeability, etc.), and at present, there has not been obtained a toner which is acceptable to practical use.
On the other hand, as the latter hardly watersoluble inorganic substance, there have been known hardly water-soluble salts such as calcium phosphate, barium sulfate, calcium sulfate, barium carbonate, calcium carbonate and magnesium carbonate; inorganic polymers such as talc and silicic acid; metal oxides such as aluminum oxide and titanium oxide; metal hydroxides such as aluminum hydroxide and ferric hydroxide; etc. There have been also proposed processes for producing a toner through suspension polymerization by using such a dispersion stabilizer (Japanese Patent Publication (KOKOKU) No. Sho 58-49863 (i.e., No. 49863/1983); U.S. Pat. No. 4,448,871; Japanese Patent Publication No. Sho 5933910 (i.e., No. 33190/1984); U.S. Pat. 4,507,378; Japanese Laid-Open Patent Application (KOKAI) No. Sho 6122354 (i.e., No. 22354/1986); etc).
In the suspension polymerization process using the hardly water-soluble inorganic substance as described above, the particle size distribution of the resultant toner can be relatively narrow. However, in order to control the particle size within a range corresponding to an actually usable toner, there is posed a problem in the control of particle size distribution such that the amount of a dispersion stabilizer to be used for such a purpose becomes relatively large and a considerable amount of fine particles are produced on the basis of the use of a surfactant (emulsifier) as dispersing auxiliary (or dispersing assistant). Furthermore, in a case where the removal of the dispersion stabilizer and the surfactant is not sufficient by the washing of the particles with an acid or water to be conducted after the polymerization, there is posed a problem such that satisfactory electrical characteristics of the toner cannot be provided. Thus, at present, there has not been obtained a toner having excellent properties which is acceptable to practical use.